The present invention relates to an optical scanning apparatus for a facsimile system or the like.
It is known in the art to provide an optical system to focus an optical image of a scan line of an original document for facsimile transmission or the like onto a photosensitive array comprising a large number of photosensitive elements such as photodiodes arranged in a line. In this manner, the scan line of the document is divided into a number of discrete bits corresponding to the photosensitive elements which produce respective electrical signals. The electrical signals may be transmitted and applied to a printing apparatus to reproduce the scan line. The entire document is scanned by translation perpendicular to the scan line so that images of successive scan lines of the document are sequentially focussed onto the photosensitive array.
At the present state of the art, it is possible to pack approximately 1800 photodiodes into a linear space of about 30mm. This number of bits is sufficient to provide satisfactory resolution of the width of a standard business size document, or about 81/2. The focussing optical system is arranged to reduce the original scan line of the document from 81/2 to 30cm.
However, such a state of the art photodiode array is quite expensive, which in some cases increases the cost of the scanning system to a prohibitive level. A prior art expedient to reduce the cost of the scanning system is to divide the scan line of the original document into two or more portions, with optical systems to focus images of the portions onto respective photosensitive arrays. The electrical signals are combined electronically to reproduce the entire scan line. The number of photosensitive elements in each array is reduced in proportion to the number of arrays provided, so that the scanning system may be implemented by two or more arrays of less elements which are lower in total cost than a single closely packed array. A commercially practical system has been implemented utilizing two arrays.
However, even photosensitive arrays comprising numbers of elements which are substantially less than the state of the art are expensive compared to optical components such as mirrors, and any number of photosensitive arrays in excess of one indicates a system which is less than optimum from the viewpoint of cost reduction.